Rebound

* Added new WHFastHelio integrator. This integrator uses the WHFast Kepler solver, but uses democratic heliocentric coordinates (WHFast itself uses Jacobi coordinates). Heliocentric coordinates are advantages if planets swap positions.

* Changes to how particle hashes are handled.

* This version removes the old SWIFTER based Wisdom-Holman routine, INTEGRATOR_WH. It wasn't working correctly for a while and the WHFast (INTEGRATOR_WHFAST) should be superior in any possible case we can think of.

* Added warning/error message system. This allows warning messages to be shown directly in iPython/python programs, rather than being shown on the console. To hide the warning messages, use a filter, e.g.
.. code:: python

with warnings.catch_warnings(record=True) as w:
warnings.simplefilter("always")
Execute a command which triggers a warning message.
The message will not show up.
* Improvements regarding the WHFast logic for hyperbolic orbis. No changes should be noticeable to users.

* Added the reb_simulation_get_serialized_particle_data function for fast access to particle data via numpy array. The full syntax is explained in the documentation. Here is a short example:
.. code:: python

import numpy as np
a = np.zeros((sim.N,3),dtype="float64")
sim.serialize_particle_data(xyz=a)
print(a)

* When loading a simulation from a binary file, REBOUND now checks if the version of the binary file is the same as the current version.
* When saving a simulation to a binary file, all the auxiliary arrays for IAS15 are now stored. This allows for bit-by-bit reproducibility in simulations that are making use of checkpoints.